Cold weather can influence fish health

If you have ever seen a fish kill, the smell and sight is something you will never forget.

Many of us have grown accustom to seeing fish kills in the summer, but during the winter months there are several natural factors such as dissolved oxygen levels, water temperature and plant material that can cause fish die-offs in local ponds too.

Fish use oxygen dissolved in the water column to breathe. Yes, as crazy as it sounds, fish do need oxygen to live underwater, and when dissolved oxygen levels become too low, fish struggle to breath. This is why the lack of dissolved oxygen is one of the main contributors to local fish die-offs.

Water oxygen levels are fluctuating constantly because they are influenced by the amount of sunlight, plant material, aquatic life and human activity taking place in and around the pond. However, warm water holds less oxygen than cold water. This fact should lead pond owners or admirers to the next logical thought, which is, my fish should be safe from oxygen-related die-offs in the winter.

This is not necessarily the case because cooler weather slows plant growth and as a result, less dissolved oxygen is produced for the fish.

Cooler temperatures tend to cool down the surface water of the pond. Interestingly, cold water is more dense than warm water and can sink to the bottom of the pond. This forces the warmer, lower oxygen water to the surface. This phenomenon is called ‘pond turnover.’ Since most fish are bottom feeders or top feeders, pond turnover can cause fish to struggle for oxygen.

Pond turnover can happen anytime, but it is especially pronounced when the temperature fluctuates drastically like we have seen during the last couple of weeks. A sign of oxygen induced stress is when fish can be observed gulping for air at the surface of the water.

Cold water fish kills can also be related to the species. North Florida’s sub-tropical climate and several years of mild winters have allowed quite a few exotic tropical fish species to become established in our area. Just as salmon are accustomed to cold water temperatures, several Florida fish like tilapia and peacock bass are acclimated to warm water temperatures.

So, if we have a cold snap that lowers water temperatures below 55°F, entire populations of these non-native tropical fish will die-off. We witnessed this locally in the winter of 2017 when cold snaps caused water temperatures to drop low enough to kill off hundreds of tilapia in ponds around Fleming Island.

You have most likely heard about algae blooms causing fish kills during the summer, but you should also know that algae are necessary components of pond ecosystems. The primary source of oxygen in a pond comes from microscopic algae or other plants. If these plants are not part of the ecosystem, it can potentially lower the oxygen levels enough to stress the fish.

If you are trying to control aquatic weeds during the wintertime, treat small areas so as to not decrease the oxygen supply. Treating too big of an area could potentially cut off the oxygen supply in your pond. Most aquatic weed control products need a specific temperature to work, so pay close attention to the product label too.

If you see a fish die-off you can report it to the Florida Fish and Wildlife Conservation Commission’s fish kill hotline at 800-636-0511. You can also contact Luke Harlow with the UF/IFAS Extension Clay County office at (904) 284-6355 for recommendations related to fish kill prevention and pond management.

There are many factors that can influence fish health but understanding the abovementioned factors can go a long way in helping you identify weather-related stress early.